The principal goal of a seismic survey is to image or map the subsurface of the earth by sending energy down into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the down-going sound energy might take many forms but is usually generated by an explosion or seismic vibrators on land, or air guns in marine environments.
During a seismic survey, the energy source is systematically positioned at a variety of locations on or near the surface of the earth above a geological structure of interest. In a typical survey, each time the source is activated it generates a seismic signal that travels downward through the earth, is partially reflected, and, upon its return, is recorded at a great many locations on the surface. The seismic signals are partially reflected from discontinuities of various types in the subsurface (including reflections from “rock layer” boundaries) and the reflected energy is transmitted back to the surface of the earth where it is recorded as a function of travel time.
The sensors that are used to detect the returning seismic energy usually take the form of geophones (land surveys) and hydrophones (marine surveys). In most cases the returning seismic energy is initially sensed as a continuous analog signal which represents amplitude variations as a function of time. The analog signals are thereafter generally quantized and recorded as a function of time using digital electronics so that each data sample may be operated on individually thereafter.
Multiple source activation/recording combinations are subsequently combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single straight line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a systematic pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
As is well known to those of ordinary skill in the art, a land survey typically uses one of two energy sources to generate the down going seismic signal: either an explosive source or a vibrational source. Of particular interest for purposes of the instant disclosure is the use of seismic vibrator. A seismic vibrator generally takes the form of a truck or other vehicle that has a base plate that can be brought into contact with the earth under control of the operator. A reaction mass (which may be on the order of two tons in weight) is driven by a hydraulic jack to produce vibratory motion which travels downward into the earth via the base plate, thereby becoming the source seismic signal. It is common to design a survey that uses multiple vibrators, each being activated simultaneously so that the recording instruments capture a composite signal with contributions from all of them. As was indicated previously, the vibrators (and receiving geophones) are generally moved together across the survey area so that recordings are generated at many different locations and a profile of the subsurface is obtained thereby.
Each excitation of the vibrator is known as a “sweep” (or, sometimes, as a “chirp”). Although many sweep patterns are possible, a common one is the “linear” sweep which is designed to vary between two frequency limits (e.g., between 5 Hz and 150 Hz) over a predetermined period of time. The amplitude of the sweep signal might either be fixed or frequency dependent, depending on a number of factors well known to those of ordinary skill in the art.
One relatively recent advance in the art of vibratory seismic data acquisition involves a technology known as high fidelity vibratory seismic (“HFVS”, hereinafter). In the HFVS method, multiple (i.e., two or more) seismic vibrators are operated simultaneously, thereby creating a complex source signal. Typically, a vibrator motion signal will be recorded during each sweep for each vibrator, and the uncorrelated geophone data will similarly be recorded and stored, with the intent of later using the recorded information to process the seismic data and to produce a subsurface image according to methods well known to those of ordinary skill in the art.
In HFVS applications, it is desirable to separate the contributions of each individual vibrator from the recorded composite signal in a multi-vibrator survey. To that end, and is well known to those of ordinary skill in the art, the sweep signals of each vibrator can be varied in such a way as to make later separation feasible. One popular approach to this variation is to use “phase encoding” which, in simplest terms, involves the application of a constant phase shift to each vibrator's signal. That is, each vibrator generates an identical signal but the phase of each is shifted by a predetermined amount with respect to the others. Consider, for example, a scenario wherein there are four vibrators. A particular phase encoding scheme in this scenario might call for the vibrator signals to be shifted, 0, 180, 90, and 90 degrees respectively.
Further, and this is particularly useful in the case of high fidelity surveys (discussed below), often multiple sweeps of the vibrators are conducted at each location. In such a case, it is common to choose a different phase encoding scheme for each sweep. Continuing with the previous example, in a multi-sweep/multi-vibrator survey, the phase encoding for the first sweep at a location might take the form identified above (i.e., phase shifts of 0, 180, 90, and 90 degrees). However, a second sweep at the same location might utilize phase shifts of 0, 90, 180 and 90 degrees respectively.
When it comes time to separate the contribution of each individual vibrator from the composite signal, it is conventional to use an inversion technique which employs either an empirically obtained measure of the source signal (ground force signal) as provided by accelerometers mounted on the vibrator structure to record accelerations of base plate and reaction mass or through the use of a “pilot” signal, a pilot signal being an idealized representation of source energy waveform.
In most cases the multiple vibrators will be positioned relatively close together (e.g., separated by a few meters or tens of meters) and usually follow a pre-designed vibrator-array pattern for the entire survey.
The fidelity of the source separation in connection with the HFVS technology depends to a large degree on the selection of an appropriate vibrator phase-encoding scheme, a good scheme being one that leads to better source separation. Better source separation, in turn, will result in an improved data quality
However, as promising as this technology might be, there is currently no satisfactory method for choosing among the potentially infinite number of combinations of sweep phase angles. There is a general feeling in the industry that the vibrators should produce signals that are orthogonal to each other (e.g., separated by a 90 degree phase rotation) but, even assuming that is a design goal, there are still a number of ways that criteria might be implemented with no clear-cut method for choosing between them to ensue that the source separation is as good as possible. Further, if the orthogonality requirement is relaxed and the phase angles of the sweeps are allowed to be non-orthogonal, there is also potentially an infinity of phase encoding schemes to choose from, which means that the problem of selecting the best phase shifts is far too complicated to approach manually.
As a consequence, in some cases a great deal of time and energy may be devoted to determining the best combination of phase angles to use in a particular circumstance. Additionally, in many cases the choice of the relative phase angles is made by viewing raw seismic records which are almost invariably contaminated with noise to the extent that it is difficult to make a clear determination of which approach is best.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of improving the quality and efficiency of acquiring HFVS data. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.